1,2-Bis(diphenylphosphino)ethane

1,2-Bis(diphenylphosphino)ethane (dppe) is a commonly used bidentate ligand in coordination chemistry. Dppe is almost invariably chelated, although there are examples of unidentate (e.g., W(CO)₅(dppe)) and of bridging behavior.^[1]

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Preparation

The preparation of dppe is conducted via the alkylation of NaPPh2 which is typically prepared from triphenylphosphine (P(C₆H₅)₃) as follows:^[2]

1. P(C₆H₅)₃ + 2 Na → NaP(C₆H₅)₂ + NaC₆H₅

NaP(C₆H₅)₂, which is readily air-oxidized, is treated with 1,2-dichloroethane (ClCH₂CH₂Cl) to give dppe:

2. NaP(C₆H₅)₂ + ClCH₂CH₂Cl → (C₆H₅)₂PCH₂CH₂P(C₆H₅)₂ + 2 NaCl

Reactions of dppe

Reduction

The reduction of dppe by lithium to give PhHP(CH₂)₂PHPh has been reported.^[3]

1. Ph₂P(CH₂)₂PPh₂ + 4 Li → PhLiP(CH₂)₂PLiPh + 2 PhLi

Hydrolysis by water gives:

2. PhLiP(CH₂)₂PLiPh + 2 PhLi + 4H₂O → PhHP(CH₂)₂PHPh + 4 LiOH + 2C₆H₆

Oxidation

Treatment of dppe with conventional oxidants such as hydrogen peroxide (H₂O₂), aqueous bromine (Br₂), etc., always produces dppeO in low yield (e.g., 13%) as a result of non-selective oxidation leading to mixtures of the starting material, the monoxide, and dioxide.^[4] Selective mono-oxidation of dppe can be achieved by reaction with PhCH₂Br to give dppeO.

3. Ph₂P(CH₂)₂PPh₂ + PhCH₂Br → Ph₂P(CH₂)₂PPh₂(CH₂Ph)⁺Br^-

This is followed by purification and alkaline catalyzed hydrolysis of the mono-phosphonium salt.

4. Ph₂P(CH₂)₂PPh₂(CH₂Ph)⁺Br^- + NaOH + H₂O → Ph₂P(CH₂)₂P(O)Ph₂

Coordination complexes of dppe

Coordination complexes of dppe, and diphosphine ligands in general, are almost entirely used as homogeneous catalysts for a wide range of reactions. Chiral diphosphines are especially important to the pharmaceutical industry^[5] for their ability to catalyze asymmetric reactions^[6] Two simple coordination complexes of dppe include Pd(dppe)₂ and Ir(dppe)₂. Pd(dppe)₂ can be prepared by reduction of Pd(II) with NaBH₄. It is most conveniently prepared, however, in situ from Pd(OAc)₂.^[4]

References

1. ^ Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensive Text, 4th ed.; Wiley-Interscience Publications: New York, NY, 1980; p.246. ISBN 0-471-02775-8
2. ^ Girolami, G.; Rauchfuss, T.; Angelici, R. Synthesis and Technique in Inorganic Chemistry, 3rd ed.; University Science Books: Sausalito, CA, 1999; pp. 85-92. ISBN 0-935702-48-2
3. ^ Dogan, J.; Schulte, J.; Swiegers, G.; Wild, B.; J. Org. Chem. 2000, 65, 951-957.
4. ^ ^{a b} Encyclopedia of Reagents for Organic Synthesis 2001 John Wiley & Sons, Ltd
5. ^ Stibbs, W. Technology & Services Business Briefing: Future Drug Discovery 2003.
6. ^ Imamoto, T. Pure Appl. Chem., 2001 Vol. 73, No. 2, 373-37